Diet of dominant demersal fish species in the Baltic Sea: Is flounder stealing benthic food from cod?

Abstract

<jats:p>Knowledge about ecological interactions between species is of paramount importance in ecology and ecosystem-based fisheries management. To understand species interactions, studies of feeding habits are required. In the Baltic Sea, there is good knowledge of the diet of cod, but little is known about the diet of flounder, the second most abundant demersal fish in the region. In this study, we investigated the diets of cod and flounder for the first time using stomach content data collected simultaneously in 2015-2017 over a large offshore area of the southern Baltic Sea. The diet of flounder was relatively constant between sizes and seasons and was dominated by benthos, with a high proportion in weight of the benthic isopod <jats:italic>Saduria entomon</jats:italic>. The diet of cod differed between seasons and showed an ontogenetic shift with a relative decrease of benthic prey and an increase of fish prey with size. Historic diet data of cod were used to explore cod diet changes over time, revealing a shift from a specialized to generalist feeding mode paralleled by a large relative decline in benthic prey, especially <jats:italic>S. entomon</jats:italic>. Flounder populations have increased in the past 2 decades in the study area, and therefore we hypothesized that flounder have deprived cod of important benthic resources through competition. This competition could be exacerbated by the low benthic prey productivity due to increased hypoxia, which could contribute to explaining the current poor status of the Eastern Baltic cod. The results of this study point to the importance of including flounder in multispecies end ecosystem models.</jats:p>

Reference state, structure, regime shifts, and regulatory drivers in a coastal sea over the last century: The Central Baltic Sea case

“…The flounder diet is spread over mussels, crustaceans, and polychaetes. Unfortunately, our data and analysis covered only the mussel component of the benthic community, and we were thus unable to fully illustrate the effect of diet overlap and competition presented in other studies (Haase et al 2020). Despite this, we believe that the poor condition of cod prior to the 1940s (Eero et al 2011) suggests low benthic food availability due to flatfish competition, since the food base (the available biomass at lower trophic levels) was much smaller during the low productive historical period compared with today (1/4 th of present levels) (Elmgren 1989), both in the benthic and pelagic domains.…”

“…Cod and flounder diets overlap (Haase et al 2020), which is one of the potentially important mechanisms for Baltic food‐web dynamics during this period. Generally, cod food items do not include a large share of mussels (Haase et al 2020), but other benthic groups may make up to 70% of the cod diet, depending on the season and cod size (Neuenfeldt et al 2020). The flounder diet is spread over mussels, crustaceans, and polychaetes.…”

“…from the long‐term simulation output (1925–2005) for the Bornholm Basin above the halocline. Bivalves, like Macoma sp., are the main diet component of Baltic flounder but do not significantly contribute to the diet of cod (Haase et al 2020).…”

Evaluating drivers of spatiotemporal individual condition of a bottom-associated marine fish

“…This highlights the need to incorporate changes in the distribution when assessing changes in the environmental conditions that cod experience. An indirect effect of declining oxygen is the potential amplification of competition with other cod and/or flounder for shared benthic prey species, such as the isopod S. entomon , due to habitat contraction caused by the expansion of “dead zones” avoided by cod (Casini et al ., 2016a, 2021; Orio et al ., 2019; Haase et al ., 2020). To address the potential effects of changes in intra- and inter-specific competition, we used predicted density of flounder and cod at the haul- and at the ICES rectangle-level, as well as S. entomon densities as covariates.…”

“…This has compromised the stock productivity to the extent that population biomass is expected to remain below safe limits despite the ban of targeted cod fisheries in 2019 (ICES, 2021a(ICES, , 2021b. Second, the Baltic ecosystem has seen a major change in the abundance of both cod and its potential competitors for the important benthic prey Saduria entomon (Haase et al, 2020;Neuenfeldt et al, 2020), the flounder complex (European flounder Platichthys flesus and Baltic Flounder Platichthys solemdali) (Orio et al, 2017), as well as an increase in abundance of its main pelagic prey species (sprat Sprattus sprattus and herring Clupea harengus) in northern areas where cod are scarce (Casini et al, 2011;Eero et al, 2012;ICES, 2021a). Lastly, the irregular inflows of saline and oxygenated water from the North Sea together with a long residence time (25-30 years) are features that have contributed to making the Baltic Sea the largest anthropogenically induced hypoxic area in the world (Carstensen et al, 2014).…”